The present invention relates to a liquid crystal display (LCD), and more particularly, to an LCD having means for orientation and optical phase retardation.
Since surface orientation of a liquid crystal is an important method in deriving a uniform orientation of liquid crystal molecules without any affect due to an electric or magnetic field nor intense light, generally, among the core manufacturing techniques of LCDs, the surface orientation of liquid crystal in a thin film state is very important. Various known methods include the vacuum deposition of silicon oxide, the rubbing of a polymer film (e.g., polyimide, nylon, Teflon and polyvinyl alcohol), the manufacture of grooves, the manufacture of a Langmuir Blodgett polyimide layer, and the adsorbing of surfactant to a surface layer. Considering mass productivity and reliability, the above polyimide rubbed layer is widely used (see U.S. Pat. Nos. 3,834,792 and 3,994,567).
One important physical variable in liquid crystal orientation is the pre-tilt angle at the orientation surface of a liquid crystal molecule. The pre-tilt angle in a twisted nematic (TN) LCD is relatively small (about 1.degree. to 2.degree.) and, in a super twisted nematic (STN) LCD, is relatively large (about 4.degree. to 8.degree.). The orientation pre-tilt angle is an important variable in the maximization of the liquid crystal twist angle in STN LCDs, the minimization of a critical voltage value during operation, and the optimization of reaction velocity considering electro-optic characteristics.
In a surface-stabilized LCD utilizing ferroelectric liquid crystal (FLC), the pre-tilt angle of the liquid crystal must be about 20.degree. or greater. Achieving such a large angle is known to be difficult in the implementation of the conventional polyimide rubbing method.
Orientation of liquid crystals utilizing a rubbed polymer film is determined by an interaction through electromagnetic force between liquid crystal molecules and orientation material molecules and by the minimum state of elastic free energy of liquid crystal due to grooves produced by rubbing. The surface anchoring strength obtained by the above rubbing is generally strong and the intensity control thereof is difficult. Here, if the surface anchoring strength is too strong, defects such as contrast deterioration and the destruction of the liquid crystal orientation state by physical impact occur. Moreover, the partially non-homogeneous alignment due to grooves formed after rubbing becomes a factor for contrast deterioration.
In order to overcome the limitation of the orientation layer manufactured using a polymer as described above, an LCD in which liquid crystal molecules are aligned using a liquid crystalline polymer orientation layer is reported. The schematic cross-sectional view of such an LCD is illustrated in FIG. 1.
Referring to FIG. 1, transparent electrodes 12 made of such material as indium-tin oxide (ITO) are coated on a pair of substrates 11, respectively. Liquid crystalline polymer orientation layers 13 are formed on the surfaces of the ITO electrodes 12, and spacers 15 for keeping a constant thickness and liquid crystal 14 are filled in the space between the thus-formed liquid crystalline polymer orientation layers.
U.S. Pat. No. 4,469,408 discloses a technique using a liquid crystalline polymer film as the liquid crystal orientation layer. Here, liquid crystalline polymer is coated on the transparent electrodes, and then an electric field is applied to change the orientation of the liquid crystalline polymer molecules to thereby change the orientation state of the liquid crystal.
U.S. Pat. No. 5,067,797 discloses an LCD manufactured by attaching a thin polymer film obtained by dissolving polymer material in a solvent and then dispersing the polymer in water. The dispersed film is then drawn and taken up in a given direction to thereby align polymer chains in a given direction. More rubbing or pressing in one direction can be applied by means of a roller when liquid crystalline polymer film is used. At this time, since the thickness of the orientation layer is as thin as 0.1 .mu.m, the optical phase retardation of the transmitted light is negligible during operation.
However, in TN, STN, FLC and electrically controllable bi-refringence (ECB) LCDs, an optical phase retardation film is used to enhance contrast. One or two films are attached on the outside of the substrates, and a protection layer, waterproofing layer or the like should be attached thereon. Finally, the overall thickness of the device becomes 100 .mu.m or more and the manufacturing method thereof is very complicated.